Document verifier using photovoltaic cell with light sensitive bars

ABSTRACT

A dollar bill verifier including a photovoltaic cell having light sensitive bars approximately the same width as the black grid lines on the bill. An amplifier receives the output of the cell and couples it through a potentiometer to a Schmitt trigger. The Schmitt trigger is coupled through a differentiator, integrator and potentiometer to a switch.

United States Patent Robert T. Bayne;

James S. Nawa, Santa Barbara, Calif. 837,060

June 27, 1969 Feb. 16, 1971 Standard Change-Makers, Inc. Indianapolis,Ind.

Inventors Appl. No. Filed Patented Assignee DOCUMENT VERIFIER USINGPHOTOVOLTAIC CELL WITH LIGHT SENSITIVE BARS 12 Claims, 14 Drawing Figs.

Int. Cl. ..G 01n 21/32 Field of Search 250/219 (1), 219 (N6), 219(F,DOC), 220 (SP), 206, 214, 211 (J); 356/71; 340/149, 149 (A); I356/209-21 l; 209/(PMSD); 194/4 [56] References Cited UNITED STATESPATENTS 3,031,076 4/1962 DeClaris et a1. 3S6/71X 3,158,748 11/1964Laycak etal. 250/219 3,188,475 6/1965 Miller 250/211 3,206,606 9/1965Burgo et a1. v. 250/209X 3,370,986 2/1968 Amsterdam et a1 250/211XPrimary Examiner-Walter Stolwein Attorney-Woodard, Weikart, Emhardt &Naughton ABSTRACT: A dollar bill verifier including a photovoltaic cellhaving light sensitive bars approximately the same width as the blackgrid lines on the bill. An amplifier receives the output of the 'celland couples it through a potentiometer to a Schmitt trigger. The Schmitttrigger is coupled through a differentiator, integrator andpotentiometer to a switch.

TRIGGER MILLIVOLTS PATENTEUFEBIGIBH 3564.268 r j SHEET'2UF4 gaiz vvuTIME INVENTORS' ROBERT 7. BHVNE BY \/A MES 5. NA WA a/M M W29- ATTORNEYS PAIEN'IED FEB 1 s 1971 vdurs sum 3- 0F 4 TYPI.0-

- 95 9697 98 T 'SCHM/TT' TRIGGER OUTPUT PULSE AREA 7 v ill vousJ TORTIME

F ig.8

TIME Fig.1O

Fig.12

TIME CONSTANT OUTPUT OF DlFFERENT/HTOR OF INTE G RH TOR Fig.11

INVENTORS ROBERT T 5/? YNE ATTORNEYS 'PATENTEU FEB-1 s 1971 SHEET 0 4 IINVENTORS' ROBERT "Y. BDNNE' BY JAMES SNNNA ATT ORNEYS DOCUMENT VERIFIERUSING PIIOTOVOLTAIC CELL WITH LIGHT SENSITIVE BARS BACKGROUND OF THEINVENTION 1. Field of the Invention This invention relates to apparatusfor examining paper money and like documents for genuineness.

2. Description of the Prior Art It is known in the art to optically scana predetermined area of a dollar bill to determine whether the parallellines in the predetermined area are sufficiently sharp and clear for thedollar bill to be valid. This scanning has been performed by moving atransparent mask and a piece of money to be examined with respect toeach other and in a linear direction while maintaining the money and themask close to one another or in engagement with one another. The maskhas thereon a predetermined pattern of parallel spaces and lines whichcorrespond to the spaces and lines on the money. The patterns as theymove into and out of coincidence produce light flashes the frequency andintensity of which is examined. If the lines are improperly spaced orhave rough edges, the light from the two patterns will not wink on andoff with the proper frequency and intensity and the apparatus willrefuse to recognize the money.

Such prior art devices are relatively complicated and thereforeexpensive and subject to frequent repair. Also the electronic circuitsof such prior art devices are not as selective as might be desired.

SUMMARY OF THE INVENTION One embodiment of this invention might includea document verification system comprising a light source, a photovoltaiccell formed in approximately the same pattern as the document to beverified, and means for causing relative movement between the document,the cell and the light source.

One object of this invention is to provide an improved docu mentverifier apparatus.

A further object of this invention is to provide a document verifierapparatus which is relatively simple and uncomplicated.

Another object of the invention is to provide a document verifierapparatus which is capable of great selectivity. BRIEF DESCRIPTION OFTHE DRAWING 7 FIG. 1 is a section taken along the line 1-1 of FIG. 3 inthe direction of the arrows.

FIG. 2 is a somewhat schematic front elevation of a document verifierapparatus embodying the present invention.

FIG. 3 is a partially schematic partially exploded perspective viewshowing the electrical circuit and portions of the mechanical componentsof the present invention.

FIG. 4, 5, 6, 7, 8, 9,10,", and 12 are graphical plots ofthe wave formsof the various signals at various points throughout the presentapparatus and show in serial form the manner in which the signal isoperated upon by the various elements of the present apparatus.

FIG. 13 is a partially exploded perspective view'of a further embodimentof the present invention; and

FIG. 14 is a side elevation of a portion of the structure illustrated inFIG. 13. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now moreparticularly to the drawings, there is illustrated a photovoltaic cellor photocell 15. The cell 15 is made from a flat chip of silicon whichhas been cut from a grown single crystal of silicon. During the growingprocess, the silicon is doped with suitable impurities so that theresulting crystal is a semiconductor of the P type. The crystal is thensliced into flat chips such as for example about one centimeter in areaand a few thousandths centimeters thickness. The resulting chip is ofcourse P-type semiconductor.

In order to make the surface of the silicon chip photosensitive, N-typeimpurities are diffused into the P-type chip by a gaseous diffusionprocess. The final result of this procedure is to form an NPsemiconductor junction on one surface of the silicon chip which extendsapproximately angstroms into the surface of the silicon chip. The entiresurface of the chip is now photosensitive. The next step in the processis the etching of alternating areas of the NP junction. This etchingdissolves alternate spaces on the silicon surface below the level of theNP region. The resulting raised portions of the silicon surface arestill NP regions and are still each separately photosensitive. Thus, inFIG.. I thephotovoltaic cell includes the main portion thereof 16 whichis approximately one centimeter in area and a few thousandths thick andwhich is doped with P- type impurity. The portions 17 of thephotovoltaic cell are NP semiconductor junctions where the P-typesilicon has been further doped with N-type impurity. Between andoutboard of the raised portions 17 there are located etched out portions20 which are not light sensitive. In order to protect the portions 17and the pattern made up by the light-sensitive portions 17,

a transparent cover glass 21 of perhaps 0.003 centimeters thickness issecured to the chip by epoxy or the like.

Referring to FIG. 3, the raised portions 17 are shown as having theconfiguration of straight parallel bars or lines all of which areconnected together by another raised portion 22. The raised portion 22is a plated conductor which may also be covered and protected by theglass cover 21 not shown in FIG. 3. Wire leads 25 and 26 are connectedto the conductor 22 and the P-type region 16 of the chip respectively.

Referring now to FIG. 2, there is illustrated a bill verifier 30 which,except for the modifications effected by the present invention, may beidentical to the bill verifier sold by Standard ChangeMakers, Inc. ofIndianapolis, Indiana, under Model No. 25 or 25A. This bill verifierincorporates a slidable tray 31 which moves toward and away from theviewer of FIG. 2. The user of the bill verifier places the dollar billin the recess 32 and pushes the tray 31 onto the bill verifier. When thetray has moved a certain distance into the bill verifier, a cam,indicated schematically at 35, is moved out of the way of a limit stopelement 36 which is fixed to a rod 37 in turn fixed to a cart 40.

The rod 37 is slidably mounted for movement leftwardly and rightwardlyas viewed in FIG. 2 on the frame elements 41. Acting between the cart 40and the frame element 41 is a coiled compression spring 42. When themember 35 moves out of the path of the limit stop 36, the shaft 37 isfree to move leftwardly until the limit stop 36 engages the frame 41which also acts as a limit stop. Thus, the cart 40 is caused to move adistance leftwardly in the direction of the arrow 45 a predetermineddistance.

The tray 31 is transparent. Mounted on the frame beneath the tray 31 isa light source 50 which shines upwardly through the aperture 51 thencethrough the transparent tray 3I and through the bill carried by the trayinto the cart 40. The cart 40 has mounted thereon at its lower end thephotovoltaic cell 15. The cell is spaced fairly closely to the tray 31and the dollar bill in the tray, this spacing being approximately 0.001inches in a typical installation such as the one herein described andpreferably as close as possible to the bill surface.

When the tray 31 is returned to its initial position toward the viewerof FIG. 2, the cart is also returned to the illustrated rightwardposition with the limit stop 36 being held by the member 35 in readinessfor a further cycle as above described. Thus, each time the tray 31 ispushed into the bill verifier and when it reaches a certain point in itstravel into the bill verifier, the cart is tripped so that the spring 42can move it through a predetermined distance leftwardly and the cellmoves across the bill in the direction of the arrow.

The light-sensitive bars or strips 17 on the photovoltaic cell are ofthe same size and relative spacing as the grid lines printed on themoney being examined, although the bars 17 may be somewhat thinner thanthe grid lines on the money. As the light passes through the printedgrid lines on the bill, it forms a shadow pattern on the surface of thephotocell adjacent to the bill. With the proper relative positioning ofthe bill and the photocell, the dark shadow areas will completely maskall of the light-sensitive strips of the photocell. At this point, theelectrical output will be of low voltage. When the relative position ofthe bill and photocell is adjusted so that the dark or shadowed portionsof the transmitted light pattern fall between the photosensitive stripsof the photocell, the photosensitive strips will be illuminated atgreater intensity and the voltage output of the photocell will be high.The relative linear motion produced by the cart 40 across the billcauses alternate illumination and shadowing of the photocell resultingin an alternating voltage output being produced by the photocell. Anexample of such an alternating voltage output is shown in FIG. 4.

Since the photocell is a direct light to energy converter, no powersupply or external source of voltage is required by the photocell togenerate the alternating output voltage. Typical cell outputs can rangefrom 0.5 to 1.5 millivolts peak to peak measured with a voltmeter having6,000 ohms internal resistance. Typical frequencies of alternatingvoltages generated by the photocell measure from 800 to 1,500 cycles persecond on the above described representative type of mechanicaltransport system. The frequency of photocell output, however, could varywidely from one type of mechanical transport system to another.

The output of the photocell is connected by means of the wires 25 and 26to amplifier input terminals 60 and 61 of the amplifier 62. Theamplifier 62 is of the integrated circuit operational type and has adifferential input. The function of the amplifier is to amplify thetypically 1.0 millivolt signal from the photocell to a value of I to 1.8volts (typical) at the amplifier output 65. An example of a suitableamplifier 62 might be RCA type CA3033.

Thus, the total voltage amplification provided at this point in thesystem is approximately l,200 to 2,000 times. In FIG. the wave form ofthe amplifier output is illustrated. The amplifier wave form output is afaithful reproduction of the wave form shown in FIG. 4 but is muchamplified in magnitude of voltage. The dotted line 70 in FIG. 4 and 71in FIG. 5 illustrates a typical variation in illumination over the areaof the lighted spot of the bill 72 in FIG. 3. As suggested, this causesa low frequency modulation of the wave form at the output of thephotocell. Because this modulation is in the nature of interference, itcontributes no information about bill characteristics. The amplifier isdesigned so that the maximum gain of the amplifier exists over afrequency range corresponding to the grid line frequencies of thephotocell. In this manner, the interference signal may be attenuatedwith respect to the grid line signal. It is not possible or desirable,however, to totally remove this interference signal at the present pointin the system. Consequently, the amplified signal from the photocellwill contain both interference signals and the grid line signals atpoint 65 in FIG. 3.

After being amplified, the signal is coupled into the input of a schmitttrigger 75 by means of a capacitor 76 and a potentiometer 77 so that theamount of signal transmitted into the schmitt trigger can be adjusted involtage level. The wave form of the signal at the input to the schmitttrigger is illustrated in FIG. 6 The wave form 76 shows the .minimuminput to the schmitt trigger and the wave form 78 shows the maximuminput. In other words, the difference 80 is a typical range ofadjustment by the potentiometer 77.

The characteristic of the schmitt trigger circuit is that the output isconstant in amplitude when present but is present or not presentdepending upon the voltage level of the input signal. FIG. 7 is a doublegraph which shows at the lower portion of the graph in the wave forms 81and 82 typical inputs to the schmitt trigger. At the upper portion ofthe graph of FIG. 7 the corresponding typical outputs of the schmitttrigger are shown at 85. Thus, it can be seen for the input 81 there isno output from the schmitt trigger. However, for the input 82 there isan output. It will be noted, however, that some of the various pulses 85of the schmitt trigger are on for shorter periods of time than otherpulses 85. This is determined by the amount of time that the signal 82is above the minimum on" level for the schmitt trigger which in theillustrated example is 0.5 volts schmitt input. It can be seen that thecause of the reduction in width of the signal produced in the upperportion of FIG. 7 is because of the tilt or slope of the envelope of thesignal input wave form 82. A typical tilt of this type is caused by thesuperposition of interference signals of lower frequency on the billsignal wave form as suggested in connection with the envelope 70 and 71in FIG. 4 and 5. The net result is that, while the output of the schmitttrigger is extremely constant in amplitude, the interference signalcomponents are still present through their modulation of the width ortime period of the schmitt trigger output pulses.

In order to eliminate the variable width factor in the schmitt triggeroutput pulses, the output from the schmitt trigger is fed to adifferentiator circuit 90. FIGS. 8 and 9 show the efiect of thedifferentiator circuit 90. Thus, the signal 91 is the input to thedifferentiator while signal 92 is the output. The dotted lines 92', ofcourse, indicate that the upper and lower wave forms are timesynchronized. It will be noted that, even though the various schmitttrigger output wave forms 95, 96, 97, and 98 have decreasing widths,still the outputs 100, 101, 102 and 103 are of equal amplitude similarlyto the schmitt trigger output 91.

To review, the bill signal has been applied to the amplifier containingboth grid line and interference signals. The amplifier has increased thevoltage magnitude of the bill signal and the interference signals andapplied them to the schmitt trigger. The schmitt trigger a has made allof the voltage wave forms of the amplified bill signal of the samemagnitude or height. It has therefore, removed part of the interferencesignal but a component still exists due to the variable width factor ofthe schmitt trigger output. The differentiator circuit then removes thevariable width factor or interference from the amplified bill signal.Thus, the bill signal has now been cleaned up" or scrubbed" and is readyto produce a grid line count or totalization at the integrator.

The output of the differentiator is coupled into an integrator 110. Theintegrator is designed with a time constant. During the time of the timeconstant, the DC output voltage of the integrator circuit isproportional to the number and the area of voltage pulses at its input.FIG. 10 shows an enlarged view of the differentiator output and showsthat each pulse has a certain area which is determined by the constantheight or amplitude 111 of the pulse and by the width or time periodofthe pulse 112. Thus, the'output of the integrator DC voltage isapproximately twice as high for 20 pulses ofa constant area as it is for10 pulses of the same area, provided, of course, that the natural timeconstant of the integrator is much longer than the span of time occupiedby the 20 pulses. FIGS. 11 and 12 show the above described action ofadding or totalizing pulses. The dotted lines betweenthe upper and lowertime axes indicate time synchronization. Thus, in FIG. 12, if theintegrator output voltage level required for bill acceptance is thevoltage indicated at 121 which corresponds to 8 pulses, then 8 pulses atthe input to the integrator would cause acceptance while, for example, 5pulses indicated at 122 would not.

The output of the integrator circuit 110 is connected to a final triggeror switch 120 through a potentiometer 121. The final trigger 120 mayconsist of a silicon controlled switch or SCS. The input 122 to thefinal trigger is connected to the gate electrode of the SCS. When thegate electrode becomes positive by about 0.4 to 0.6 volts, the SCScomponent conducts giving a go or accept" signal which is delivered tothe output 125. The potentiometer 121 connected between the integratorand the final trigger make possible adjustment of the voltage level atwhich the SCS or final trigger is fired.

While it has not been been previously stated, each grid line existing onthe bill and scanned by the photocell produces one voltage pulse at theinput to the amplifier. These amplified pulses are made of constantamplitude and constant width by action of the schmitt trigger and thedifferentiator circuits. The integrator sums the'se pulses, giving atleast approximately equal weight to each pulse. The integrator output DClevel is,

therefore, an exact function of the number of grid line pulses scanned.Amplitude and frequency of the grid line pulses do not affect the count.Thus, by adjustment of the potentiometer 121, the final trigger can bemade to give an accept" signal at any desired number of grid linesdetected. For example, the potentiometer 121 may be adjustedto give anaccept signal after grid lines are added up by the integrator or, ifdesired, after grid lines are detected by the integrator. Thus, thepotentiometer 121 is a means of varying the selectivity of the entiregrid line detection system.

The potentiometer 77 may be adjusted so that the schmitt trigger 75 isoperating for the minimum grid line signal generated by a worn, old andwrinkled bill of genuine printing. As mentioned above, the schmitttrigger will not operate unless it receives a signal of a certainminimum value. The potentiometer 77 can be adjusted so that the schmitttrigger will not operate at all for a great amount of obviously bogusmoney such as play money, photostats, poor quality counterfeits and thelike. Thus, no signal will be generated for such items and no signalwill be passed on to the differentiator for such items.

Thus, the net result of the structure described above is to perform thefollowing three checks for bill authenticity:

l. ls a grid line signal generated'f'lhis check is performed by thephotocell and the spacing and width of the light-sensitive areas of thephotocell.

2. If a grid line signal is generated, is it of enough amplitude to betypical of either an old or a new genuine billZThis is accomplished bythe schmitt trigger and the threshold adjustment of the potentiometer77.

3. If the grid line signal is of sufficient amplitude so that it mightbe genuine, what is the total grid line count over the length of thescan? ls it of sufficient number so that it is typical of agenuine'bill? This check is accomplished by the combination of theschmitt trigger, differentiator, integrator, the adjustment of thepotentiometer 121 and the final trigger 120.

Referring now to FIGS. 13 and l4,there is illustrated an alternativeembodiment of the invention which is identical to the apparatusdescribed in FIGS. 1l2, except as described below. In place of the coverglass'21, a fiber optics tube or conduit 200 is used between thedocument or bill 201 and the photovoltaic cell 202. The tube 200 may be,for example, Model No. lC-b 200-6 image conduit manufactured by TheAmerican Optical Company of Southbridge, Massachusetts, although thetube is cut and polished to a shorter length than the usual 6 -inchlength of this model number. As shown one end of the image conduitcontacts the bill 201 while the other end is in contact with or closelyspaced to the cell 202. The end surfaces of the image conduit areoptically fiat and polished. The image conduit is made up of at least70,000 separate fibers for each one-fourth inch of conduit diameter.Each fiber is separately clad to individually transmit a light spotthrough the conduit. The size of each light spot is very small (about 12microns). The grid line image is thus transmitted up the conduit to thecell surface.

The advantages of replacing the cover glass 21 with the conduit 200 arethat the cell 202 may be located within a protective housing 205 whichis identical to or corresponds to the cart 40. The housing 205 shieldsthe cell from mechanical shock, vibration, moisture and other foreignmatter which might be picked up from the bill surface. The housing alsoprotects the cell from external electrical contact. Also the conduit 200prevents substantial diffusion of light in the area between the cell andthe document so that a sharp clear image is delivered to the cell.

It will be evident from the above description that the present inventionprovides an improved document verifier apparatus. It will also beevident that the document verifier apparatus of this invention isrelatively simple and uncomplicated and is capable of great selectivity.

We claim:

1. A document verification system comprising a light source; aphotovoltaic cell'formed in approximately the same pattern as thedocument to be verified; and means for causing relative movement betweenthe document, the cell and the light source, said document including aplurality of bars, said photovoltaic cell also including a plurality ofsimilar bars of light-sensitive material of the same relative size andspacing as the bars of the documents, said bars being separated bynonlight-sensitive material.

2. A document verification system as defined in claim 1 wherein saidbars are parallel.

3. A document verification system as defined in claim 1 additionallycomprising a fiber optics image conduit positioned between the lightsource and the cell and conducting an image of the document from thedocument to the cell.

4. A document verification system as defined in claim 2 wherein saidcell includes a fiat chip of silicon doped with P- and N-typeimpurities, said flat chip being doped with oneof said types ofimpurities therethrough, said chip having a photosensitive surfacethereon formed by the other of said types of impurities fused into theone type impurity chip, said chip being etched at selected portions ofsaid surface to remove portions of said photosensitive surface providings'aid pattern of photosensitive material.

5. A document verification system as defined in claim 2 additionallycomprising first means for receiving the output of said cell and forconverting those portions thereof which are above a given amplitude intopulses of constant voltage of varying time duration which is determinedby the time that said portions are above said amplitude, second meansfor differentiating the output of said first means to produce for eachinput pulse an output pulse that is constant in amplitude and timeduration, integrating means for counting the output pulses of saiddifferentiating means and producing an output directly proportional tothe number of second means output pulses, and switch means receiving theoutput of said integrating means and arranged to be turned on by anoutput of given amplitude from said integrating means.

6. A document verification system as defined in claim 5 wherein saidmeans for causing relative movement comprises a spring and a pair oflimit stops and a cart, said cell being mounted on said cart, said limitstops'being spaced apart a fixed distance, one of said limit stopsnormally acting to hold said cart at a first position, said'one stopbeing movable to permit said cart to move from said first position to asecond position under the action of the spring, said first and secondpositions being a given distance apart so that the travel of said cartbetween said positions is always the same distance.

7. A document verification system as defined in claim 6 additionallycomprising a fiber optics image conduit mounted on said cart andpositioned between the light source and the cell and conducting an imageof the document from the document to the cell.

8. A document verification system as defined in claim 5 additionallycomprising a potentiometer coupling the output of said cell to the inputof said first means and making possible adjustment of said givenamplitude of the first means so that said first means will respond to asignal of the amplitude produced by a worn old genuine document but willnot respond to lesser amplitudes.

9. A documentverification system as defined in claim 8 additionallycomprising a potentiometer coupling said integrating means to saidswitch and making possible adjustment of the number of pulses from saidintegrating means required to turn on said switch.

10. A document verification system comprising a photovoltaic cell, alight source, said photovoltaic cell including means for comparing apattern of the document with the document itself to provide avariationin light to said cell, first means for receiving the output of'said celland for converting those portions thereof which are above a givenamplitude into pulses of constant voltage of varying time duration whichis determined by the time that said portions are above said amplitude,second means for differentiating the output of said first means toproduce for each input pulse an output pulse that is consaid first meanswill respond to a signal of the amplitude produced by a worn old genuinedocument but will not respond to lesser amplitudes.

12. A document verification system as defined in claim 11 additionallycomprising a potentiometer coupling said integrating means to saidswitch and making possible adjustment of the number of pulsesfrom saidintegrating means required to turn on said switch.

1. A document verification system comprising a light source; aphotovoltaic cell formed in approximately the same pattern as thedocument to be verified; and means for causing relative movement betweenthe document, the cell and the light source, said document including aplurality of bars, said photovoltaic cell also including a plurality ofsimilar bars of light-sensitive material of the same relative size andspacing as the bars of the documents, said bars being separated bynon-light-sensitive material.
 2. A document verification system asdefined in claim 1 wherein said bars are parallel.
 3. A documentverification system as defined in claim 1 additionally comprising afiber optics image conduit positioned between the light source and thecell and conducting an image of the document from the document to thecell.
 4. A document verification system as defined in claim 2 whereinsaid cell includes a flat chip of silicon doped with P- and N-typeimpurities, said flat chip being doped with one of said types ofimpurities therethrough, said chip having a photosensitive surfacethereon formed by the other of said types of impurities fused into theone type impurity chip, said chip being etched at selected portions ofsaid surface to remove portions of said photosensitive surface providingsaid pattern of photosensitive material.
 5. A document verificationsystem as defined in claim 2 additionally comprising first means forreceiving the output Of said cell and for converting those portionsthereof which are above a given amplitude into pulses of constantvoltage of varying time duration which is determined by the time thatsaid portions are above said amplitude, second means for differentiatingthe output of said first means to produce for each input pulse an outputpulse that is constant in amplitude and time duration, integrating meansfor counting the output pulses of said differentiating means andproducing an output directly proportional to the number of second meansoutput pulses, and switch means receiving the output of said integratingmeans and arranged to be turned on by an output of given amplitude fromsaid integrating means.
 6. A document verification system as defined inclaim 5 wherein said means for causing relative movement comprises aspring and a pair of limit stops and a cart, said cell being mounted onsaid cart, said limit stops being spaced apart a fixed distance, one ofsaid limit stops normally acting to hold said cart at a first position,said one stop being movable to permit said cart to move from said firstposition to a second position under the action of the spring, said firstand second positions being a given distance apart so that the travel ofsaid cart between said positions is always the same distance.
 7. Adocument verification system as defined in claim 6 additionallycomprising a fiber optics image conduit mounted on said cart andpositioned between the light source and the cell and conducting an imageof the document from the document to the cell.
 8. A documentverification system as defined in claim 5 additionally comprising apotentiometer coupling the output of said cell to the input of saidfirst means and making possible adjustment of said given amplitude ofthe first means so that said first means will respond to a signal of theamplitude produced by a worn old genuine document but will not respondto lesser amplitudes.
 9. A document verification system as defined inclaim 8 additionally comprising a potentiometer coupling saidintegrating means to said switch and making possible adjustment of thenumber of pulses from said integrating means required to turn on saidswitch.
 10. A document verification system comprising a photovoltaiccell, a light source, said photovoltaic cell including means forcomparing a pattern of the document with the document itself to providea variation in light to said cell, first means for receiving the outputof said cell and for converting those portions thereof which are above agiven amplitude into pulses of constant voltage of varying time durationwhich is determined by the time that said portions are above saidamplitude, second means for differentiating the output of said firstmeans to produce for each input pulse an output pulse that is constantin amplitude and time duration, integrating means for counting theoutput pulses of said differentiating means and producing an outputdirectly proportional to the number of second means output pulses, andswitch means receiving the output of said integrating means and arrangedto be turned on by an output of given amplitude from said integratingmeans.
 11. A document verification system as defined in claim 10additionally comprising a potentiometer coupling the output of said cellto the input of said first means and making possible adjustment of saidgiven amplitude of the first means so that said first means will respondto a signal of the amplitude produced by a worn old genuine document butwill not respond to lesser amplitudes.
 12. A document verificationsystem as defined in claim 11 additionally comprising a potentiometercoupling said integrating means to said switch and making possibleadjustment of the number of pulses from said integrating means requiredto turn on said switch.